Chrysotile is the most commonly used and most toxic asbestiform mineral. It is hypothesized that the elemental nature of the fibers plays an important role in the mediation of disease consequent to inhalation. It also has been proposed that phagocytosis of asbestos fibers by pulmonary macrophages is an essential step in the pathogenesis of asbestosis. Our earlier studies established the elemental ratio of magnesium to silicon in inhaled fibers. Now we have used this knowledge to determine the percentage of asbestos-containing pulmonary macrophages lavaged from animals after aerosol exposure. In addition, we have learned that subtle, but consistent alterations in macrophage morphology, function and enzyme secretion are present only 48 hrs. after a 1-hr. exposure to chrysotile asbestos. By 48 hrs. after a 1-hr. exposure to asbestos, macrophages have migrated to alveolar duct bifurcations where the fibers were deposited initially and where early asbestos-induced lesions are known to develop. Significant accumulations of macrophages are found on over 90% the bifurcations.